The present invention relates to an image processing apparatus for emphasizing the color of an image.
These days, the video technique is used in a wide variety of applications. For example, in the medical field, an electronic endoscope having an image pickup device combined therewith is used for observing a diseased portion of a body cavity. Instead of being observed directly by the naked eye, the image of the diseased portion is picked up by the image pickup device, and is diagnosed on the basis of the image displayed on the monitor.
The hue of the image of a living body obtained in this manner is reddish and the change in saturation throughout is rather slight. Therefore, when a doctor, for example, observes the image, for the purpose of diagnosis he must be highly skilled in order to be able to detect a diseased portion on the basis a slight difference in hue and saturation.
An example of a conventional image processing apparatus for emphasizing the color of an image is described in "Digital Processing of Endoscopic Color Images", Optics Communications, vol. 55, No. 4, pp. 242-pp. 247. Assume an original image having a color distribution as indicated on the chromaticity coordinate plane of FIG. 1A. When only saturation components of the original image are emphasized, the distribution shown in FIG. 1B is obtained. When only hue components relative to point a representing a chromaticity average are emphasized with respect to point a as its center, a distribution such as is shown in FIG. 1C is obtained. The position of point a' in FIG. 1B differs from that of point a of FIG. 1A, which the position of point a" of FIG. 1C is the same as that of point a of FIG. 1A. Namely, in FIG. 1B showing saturation emphasis, the distances between the respective points and the origin of the coordinate plane are amplified, while in FIG. 1C, the relative coordinates of eight points with respect to point a are amplified in accordance with their distances from point a.
In the conventional image processing apparatus as described above, assume that the color distribution of an original image is as shown in FIG. 2A. In this case, when only the hue components relative to point a representing a chromaticity average are uniformly emphasized, points b and c are moved to different quadrants, as is shown in FIG. 2B. Accordingly, the tone of point c' is changed and point b' becomes a complementary color. When such an excessive color emphasis is performed, the emphasized image lacks the three color components of the portion of the original image which is to be emphasized. As a result, the color reproduction is poor.
Assume that point a and points therearound--that is, a total of nine points--represent a background image (e.g., an endoscope image of the stomach wall) of an original image. When the points which represent the background image and points b and c, which do not, are subjected to amplification and are thus emphasized, the background image is emphasized to cause a decrease in the S/N ratio, which result in considerable variance in the background hues. As a result, the original purpose of emphasizing points b and c with respect to the background image cannot be achieved, and the level of contrast of the entire image becomes low. This is particularly inconvenient when a portion of a medical image whose color is slightly different from its background, such as a diseased portion, is subjected to emphasis for identification.